At the end of pregnancy, the myometrium becomes extremely sensitive to oxytocin (OT) as result of a dramatic increase in the number of OT receptors (OTR), indicating an important role for OTR in the process of labor. There are no studies in sheep in which the physical properties and histological distribution of OTR are evaluated in relation to parturition. Also, no studies have been performed in any species to simultaneously examine the distribution of OTR at the messenger RNA (mRNA) as well as the protein levels in the same tissues and correlate those changes with the patterns of myometrial activity that occur at labor. In the present studies, we have used a polyclonal anti-OTR antibody and Western blot analysis to determine the apparent molecular mass of ovine OTR in late pregnant sheep myometrium and endometrium. We also examined the distribution of OTR mRNA and protein expression in the intact myometrium and endometrium and in individual cultured cells using in situ hybridization and immunocytochemistry. The expression of OTR and its mRNA has been correlated with the patterns of activity observed in the pregnant sheep myometrium. Western blot analysis of myometrial and endometrial extracts revealed a major form of OTR with an approximate molecular mass of 66 kDa. Both immunocytochemistry and in situ hybridization localized OTR and its mRNA in myometrial cells and glandular cells of the endometrium. Increased OTR and its mRNA expression in the myometrium and endometrium were correlated with the occurrence of myometrial contractions. OTR was also demonstrated by immunocytochemistry in the smooth muscle of myometrial blood vessels. Localization of OTR and its mRNA in pregnant sheep myometrial cells is consistent with the hypothesis that OTR plays an important role in regulating myometrial contractility. Positive staining of OTR in endometrial glandular cells supports the view that OT is involved in PG production by the endometrium in late pregnancy. Increased expression of OTR and its mRNA in the myometrium during labor further indicates that changes in tissue OTR play a significant role in the mechanism of parturition. Increased expression of OTR and its mRNA in endometrium may relate to the role of OT in regulating PG production by the endometrium during labor.
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